Clockwise block rotations along the eastern border of the Cordillera de Domeyko, Northern Chile (22°45′–23°30′S)

Abstract

We report new paleomagnetic results from a study of 38 sites in Cretaceous to Early Tertiary red beds and volcanic rocks belonging to the Purilactis Group, which outcrop along the western border of the Salar de Atacama Basin. After detailed thermal demagnetization, characteristic directions were determined for 32 units. In most cases, red bed sediments from the lower members of the Purilactis Group have a well-defined normal polarity magnetization probably carried by hematite forming an early diagenetic cement. A large decrease in the dispersion of the paleomagnetic directions upon tilt correction demonstrates that this magnetization is a pre-tectonic magnetization. We interpret the dominant occurrence of the normal polarity direction as evidence for acquisition of the magnetization during the Cretaceous normal polarity superchron. Large deviations of the paleomagnetic declinations from the expected ones for stable South America provides new evidence for clockwise tectonic rotations associated with Tertiary deformation in the Cordillera de Domeyko. These data confirm that clockwise tectonic rotations are one of the most significant structural characteristics of the north Chilean Andes. This study, however, indicates spatial variation in the magnitude of the rotation with rotations >60° in the Cerro Totola area. These rotations have occurred in conjunction with transpressional deformation that affected large tracts of the Cordillera de Domeyko during Eocene deformation. The systematic observation of clockwise rotations contemporaneous with sinistral displacements in the Cordillera de Domeyko can be explained by shear-traction at the base of the brittle crust.